When moving the light spots formed on an optical disc by light beams so as to be focused there, the sliding motor and the drive section of the objective lens are fed with an electric current. At this time, the direction in which the sliding motor and the objective lens are intended to be driven to move and the actual moving direction of the light spot do not necessarily agree with each other because of the backlash of the sliding motor and the objective lens relative to their proper motions. Therefore, it is necessary to provide a device for constantly detecting the direction in which the light spots cross the tracks and keep it operating whenever the sliding motor and the objective lens are driven to move.
FIG. 1 of the accompanying drawings schematically illustrates the principle of a known method of judging the moving direction of the light spot formed on a conventional optical disc having grooves that are formed along the lands of the optical disc without differentiating the depths of the grooves. The known method will be discussed in detail below by referring to FIG. 1.
Of the graphs in FIG. 1, a shows the profile of the grooves of the known optical disc and b shows the waveform of the difference signal obtained from a pair of reflected light detection signals of a light spot that is formed on the optical disc, while c shows the waveform of the sum signal of the detection signals. In FIG. 1, the horizontal axis represents the radial distance on the disc for all the graphs while the vertical axis represents the depth of the grooves for the graph a and the signal amplitude for the graphs b and c.
The difference signal as used herein refers to the difference between the first light detection output A obtained from the light spot formed on the optical disc by a light beam so as to cover a pair of grooves having a same depth and sandwiching a land as the light beam of the light spot is reflected and deflected by one of the pair of grooves and the second light detection output B obtained from the same light spot as the light beam is reflected and deflected by the other groove, or A−B, which is a push-pull signal. On the other hand, the sum signal as used herein refers to the sum of the detection output A of the first light beam and the detection output B of the second light beam, or A+B.
In FIG. 1, the graphs d and e respectively show signal DP (digitalized push-pull) generated by binarizing the difference signal and signal DS (digitalized sum) generated by binarizing the sum signal. The moving direction of the light spot of the irradiated light beam can be determined by using the signals DP and DS in a manner as will be described hereinafter.
Referring now to FIG. 2 showing a D flip-flop circuit 100, the signal DP is applied to the data input terminal D of the D flip-flop circuit 100 and the signal DS is applied to the control input terminal of the D flip-flop circuit 100. If the light spot moves from left L to right R as indicated by arrow (L→R) across the grooves having a depth as shown in the graph a, the DS rises at the parts indicated by 0 and the output Q is “L” for those parts because the corresponding level of the DP is “L”. If, to the contrary, the light spot moves from right R to left L as indicated by arrow (L→R) across the grooves, the DS rises at the parts indicated by A and the output Q is “H” for those parts because the corresponding level of the DP is “H”.
Thus, if the output signal Q is “L”, the light spot is judged to be radially crossing the tracks from left L to right R. If, on the other hand, the output signal Q is “H”, the light spot is judged to be radially crossing the tracks from right R to left L. In other words, it is possible to accurately determine the moving direction of the light spot by seeing the output signal Q. This technique utilizes the fact that the phase of the DS and that of the DP and hence the phase of the sum signal and that of the difference signal, from which the signals DS and DP are generated, are differentiated by 90 degrees.
Meanwhile, a high signal recording density can be effectively achieved for optical discs by raising the track density as well as the line density. The track density can be raised either by a land and groove method of recording signals on both the lands and the grooves of the optical disc or by a shallow and deep method of using a pair of grooves including a shallow groove and a deep groove that are arranged oppositely and helically to sandwich a land at a time as disclosed in Japanese Patent Application Laid-Open Publication No. 11-296910, which was filed by the applicant of this patent application.
The method disclosed in Japanese Patent Application Laid-Open Publication No. 11-296910 of the shallow and deep method using a pair of grooves whose depths are differentiated from each other will be described below in greater detail particularly in terms of recording signals on the land. The grooves of tracks that are adjacently located on a conventional optical disc have a same depth and a same width. If the tracks are arranged at an increased pitch on such an optical disc, the spatial frequency of the tracks will exceed the MTF (modulation transfer function) of the optical disc and tracking signals will no longer be generated there. Thus, the track density of conventional optical disc is limited by the tracking inability although a higher track density may be required to improve the recording/replaying performance of the optical disc.
With the shallow and deep method, every pair of adjacently arranged grooves are made to have differentiated depths. As a result, a frequency component of ½ of the track pitch is generated to make it possible to obtain a tracking error signal. With the shallow and deep method to be used with an optical disc having two tracks, the tracks are made to have profiles that are mirror-symmetric relative to each other and therefore it is easy to make them show same recording characteristics. From this point of view, there is a clear contrast between the shallow and deep method and the land and groove method, with which different tracks show different recording characteristics because signals are recorded on different areas including lands and grooves.
By the way, with the shallow and deep method, the tracking error signal shows a unit cycle of two tracks and hence both the difference signal and the sum signal are different from those of conventional optical discs. Additionally, there may be cases where the sum signal is not obtainable depending on the depths of the two grooves, although the difference signal is always obtainable. Therefore, with an optical disc designed to be used with the shallow and deep method, it is difficult to determine the moving direction of the main light spot only from the detection signal of the reflected light coming from a single light spot on the optical disc.